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Mister Double U
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Update 2 On: Carbothermic Reduction of Calcium Sulfate - Partial Success
Hello Folks,
I did another run of CaSO4 reduction. This time I did not suck air through the reaction mix but instead blowed the air through with the pressure side
of the vacuum pump. Since there was no resistance from the wash bottles, there was a much bigger flow of air and things got very hot very quickly.
The whole reaction took just 15 minutes and was stopped, when I started smelling strong SO2 evolution.
The CaS ashes were then put into a gas generating setup and HCl was dripped on top to release the H2S. This was then bubbled through a Copper Sulfate
solution to make CuS. Until this step everything worked fine.
Next, I tried to oxidize the CuS by bubbling air through it, because the German Wikipedia states that Copper Sulfide oxidizes to the Sulfate in moist
air. I tried that for ~2 weeks without success.
After that I electrolyzed the mix with a BDD anode for another week, and this seemed to decrease the volume of the solid CuS. However, there was still
some left after that. At that point I decided to stop because the process was extremely inefficient. From the starting materials, I would have ended
up with ~30g H2SO4 if everything worked out.
Anyway, here some pics:
The gas generating setup:
Copper Sulfide after settling to bottom of flask:
Electrolysis setup (Kathode: Copper wire, Anode: Boron Doped Diamond)
Anyway, I wanted to share the results so others can improve on it.
Best Greetings!
[Edited on 4-5-2025 by Mister Double U]
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Mister Double U
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Carbothermic Reduction of Sodium Sulfate
Hello,
I did another reduction run, but this time with Sodium Sulfate instead of Calcium Sulfate. The goal was to produce elemental sulfur, which then can be
used in further steps.
250g of moist Na2SO4 were mixed with 150g charcoal (I assumed 150g dry mass of NaSO4). NaSO4 was used as fine crystals resulting from evaporation in
air over the course of some weeks. Charcoal was powdered in a mortar. Both were mixed in a little plastic drum with shaking.
The reaction setup was the same as in the previous run (terra cotta pot through which air was blowed from the bottom hole. A single coal briquette was
ignited with a blow torch and then the reaction mix was poured on top of it.
After completion of the reaction, the fused mass was dissolved in water. In theory there should have been 82g of Na2S. The S[2-] needs to be oxidized
to elemental S. I chose bleach for the job. 78g of NaOCl were required. This was added as 1l of bleach. The mix heated up to ~50C during the addition
of the bleach.
During the previous oxidation polysulfides are formed which need to be broken up to release free sulfur. This was achieved by adding 400ml HCl to the
mix and Sulfur immediately precipitated out of solution. The solution was then left undisturbed for 3 days to let the tiny S particles settle.
Results:
10.7g of moist Sulfur were obtained. This yielded 7.5g dry mass.
Theoretical would have been 33g, so the yield was 22%.
Pictures:
Polysulfide solution before acidifying:
After addition of HCl:
After sedimentation:
Yield:
Burning Sulfur and residue:
Notes:
1. The Sulfur obtained was not bright yellow and had some unknown contamination.
2. During acidification a lot of H2S was liberated which means the oxidation of S[2-] was incomplete. I assume it happened because of old bleach. This
is supported by the fact, that the solution smelled like Chlorine after 3 days of settling - In other words, Chlorate present in the bleach became
slowly converted to Cl2 and ClO2 after the acidification.
Greetings!
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MrDoctor
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I cant begin to fathom why anyone would try to source sulfur in this way if it was just for producing sulfuric acid in the end, but it still looks
like it was a lot of fun, and i suppose that sulfates often are found cheaply. God knows i throw out a lot of sodium sulfate constantly any time im
using bisulfate or just neutralizing reactions that used sulfuric acid, but over here we feed sulfur to our horses and rub it on our tomatoes, so ive
never thought about making sulfur from scratch.
I couldnt find in the german wikipedia anything about copper sulfide oxidizing in moist air, however a quick search did find me some mentions that
iron may be required to complete the process, that or, it needs to be some degree of acidic or H2S for it to happen at room temperature. Are you sure
it didnt otherwise mean heated moist air?
There is a method of producing a less effective catalyst for SO2 oxidation that is just mixed copper and iron oxides, than compared to the
conventionally used and easily poisoned, vanadium pentoxide, and i believe mixed copper-iron sulfate forms and decomposes more readily than pure
copper sulfate. It seems to be a pretty common recurring theme.
A project i want to undertake some time in the future is induction heating an iron catalyst coated in mixed iron-copper oxides, in either a quartz
test tube, or inside the flask itself where sulfur is being oxidized, just using a cheap little ZVS driver, and passing SO2 and air through that, or
possibly also H2S, since in principle, it should be able to regenerate itself if the heat is cranked up periodically, allowing it to tolerate H2S and
S.
By the way; when performing an electrolysis on H2S, you need to use a separated cell, same as when electrolyzing sulfurous acid to sulfuric.
while it takes more effort than a terracotta clay pot, using plaster to make a separator should yield much cleaner acid, theres not a lot of reactions
where the presence of calcium sulfate will be detrimental as a contaminant, and im told that the sulfate functions somewhat as a solid electrolyte in
that context too, and that sometimes you can just cast the cathode into some plaster and that works, but thats just me parroting what the smart
cookies have told me.
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Mister Double U
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MrDoctor,
Here the quote from the German Wikipedia:
"In feuchter Luft wird das Erz Kupfer(II)-sulfid zu Kupfersulfat oxidiert."
-> "In moist air the ore Copper(II)-sulfide becomes oxidized to Copper Sulfate"
https://de.wikipedia.org/wiki/Kupfer(II)-sulfid
You are right, it was a lot of hassle to get that little bit of Sulfur. So, if you have access to Sulfur this is no more than a fun project. However,
there are many places where Sulfur is not readily available. So, my thinking is that it would be nice to be able to make H2SO4 from easily accessible
Sulfur compounds - which are Sulfates, especially CaSO4. This should in theory be convertible to Na2SO4 by boiling in Na2CO3 solution (like how Sodium
Hydroxide was made from Sodium Carbonate and Calcium Hydroxide). This should work because CaSO4 is ~300 times more soluble than Calcium Carbonate
(have not tried it myself).
Concerning the gypsum diaphragm, I have tried that once. Molded it in the shape of a cup with wall strength 1.5cm. After normal drying I put it in the
oven at 250C I believe. It gave up after a day or so. That does not mean it can't be done, but I didn't get it right. A flowerpot usually works nice
in acidic media. Important is that things are acidic on both sides. Otherwise, if for example on the cathode side is a basic pH and on the anode side
is an acidic pH then the OH[-] and H3O[+] ions cancel each other out in the diaphragm and conductivity drops to almost zero.
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MrDoctor
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Quote: Originally posted by Mister Double U  |
Concerning the gypsum diaphragm, I have tried that once. Molded it in the shape of a cup with wall strength 1.5cm. After normal drying I put it in the
oven at 250C I believe. It gave up after a day or so. |
you arent supposed to bake plaster, not normal plaster anyway. however no matter, a solution to this problem is to paint the walls of the cell with
PVC cement. the cell should be made by moulding the electrode chambers with a solid object then pouring your plaster, or apparently, ivory cement, in.
so you place 2 or 3 items into your container, then pour the membrane casting material, producing 2 or 3 chambers, they should all be totally isolated
from each other, so including the bottom.
the PVC cement is a viable membrane material in of itself, but it needs to be applied rather thickly to be any good, and treated, but for this, its
great, its literally just being a chem resistant glue keeping the walls in tact. Now with a seperated cell, SO2 electro oxidation is easy. In this
situation too, it might actually be desireable to use a H2SO4+S->2SO2 reaction to supply pure sulfur dioxide on demand, the only drawbacks for a
sulfate-doubler like this is it spits out a bit of wet sulfur and acid vapor which is potentially very problematic for a catalyst, but fine if its
just getting bubbled into a sulfuric acid cell.
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Varungh
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| Quote: Originally posted by Fulmen | I'm seriously considering buying a 25kg bag of sulfur. I can get it in 1/2kg quantities from a home gardening supplier, but at a obscene price. Even
including shipping the bag will be cheaper than 2kg from home garden supplier.
I just need to convince a farming friend that he won't get on any terrorist watch list by ordering it. Shouldn't be a problem, as he's producing
ecological food and sulfur is approved for this. It actually looks like a very versatile compound that can be used both for soil improvement and as a
fungicide.
The process will most likely be the contact process. Vanadium is the typical catalyst, but IIRC platinum could also be used. It's said to be sensitive
to arsenic, but fertilizer grade sulfur should be pretty low in that so it might be worth a look. Perhaps a broken automotive catalyst could work?
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Maybe finely divided nickel can work.it does seem plausible.I will try finding more info on the topic.someone supposedly ghit it working with nickel
and iron powder.
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pots-o-potash
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I've been digging around on ammonium sulfate and ammonium bisulfate decomposition pathways and, from my reading: with the multiple decomposition
stages and ranges of ammonium bisulfate, sulfuric acid and sulfur trioxide can indeed form within certain ranges and as part of a complex mix of
equilibrium reactions. The full decomposition to a mixture of NH3, SO2, N2, and water involves a redox reaction between SO3 (or H2SO4) and ammonia,
and this is accelerated by catalysts, and thus favored more heavily (and at less extreme temperatures) than when catalysts are absent. Additionally,
some references to decomposition pathways don't even mention the formation of ammonium pyrosulfate as a possible dehydration product (but do mention
evolution of ammonia and formation of H2SO4).
H2SO4 + ammonia formation is favored in the roughly 300C to 350C range, where H2SO4 vapors (and gas, above its boiling point) form as part of that
equilibrium. (not explicitly mentioned in text, but fractional distillation within that temperature range could yield ammonia and H2SO4 in separate
fractions).
If NH3 is driven off and H2SO4 is allowed to reflux, the equilibrium will be reached at some point with no further NH3 being driven off (and a mixture
of NH4HSO4 with smaller portion of H2SO4 should be present)
Above 350C, NH3 + SO3 + H2O becomes an increasingly competing products (which makes sense given H2SO4's decomposition temp range), and somewhere over
400C the SO2 + N2 + H2O redox reaction products enter the picture. (favored by catalysts or impurities acting as catalysts)
Not that these are viable or practical routes to H2SO4 or NH3 production (outside of possible niche industrial recovery/recycling processes), but
worth noting nevertheless. Potentially more relevant is the context that the commonly cited decomposition of NH4HSO4 to NH3 + SO2 + N2 + H2O at some
lower temperature ranges is (apparently) extremely dependent on the presence of catalysts that thus favor oxidation of ammonia and reduction of SO3.
That and there is no simple staged progression of the decomposition, but multiple competing reactions in equilibrium with the dynamics shifting as
temperature increases.
I don't see specific reference to it, but I would also guess that evaporation or boiling/offgassing of the volatile products would be aided by vacuum
distillation, thus favoring a larger quantity of those products at their favored temperature range with fewer competing reactions. (ie it may be
possible to vacuum distill anhydrous sulfuric acid at around 280-300C provided you could arrange a vacuum assisted fractional distillation set-up)
Also worth noting would be the initial equilibrium reaction when H2SO4 + NH3 forms around 300C (some references to 308C), which means if the reaction
is stopped there and the salt-acid mixture is allowed to solidify, it will have some quantity of free sulfuric acid present.
See:
https://www.e3s-conferences.org/articles/e3sconf/pdf/2024/12...
(Experimental Study of The Growth of Ammonium Bisulfate Ash
Deposition at Different Wall Temperatures and Heat Transfer
Characteristics)
and:
http://2011cpr.zju.edu.cn/attachments/2023-02/01-1676001363-...
(Experimental study on the effects of reheat temperatures on the ammonium bisulfate and ash blend deposition)
Attachment: Experimental Study of The Growth of Ammonium Bisulfate Ash e3sconf_icaeer2024_01002.pdf (487kB)
This file has been downloaded 70 times
Attachment: Experimental study on the effects of reheat temperatures on the ammonium 01-1676001363-12052.pdf (4.6MB)
This file has been downloaded 78 times
Attachment: Experimental Study of The Growth of Ammonium Bisulfate Ash e3sconf_icaeer2024_01002.pdf (487kB)
This file has been downloaded 52 times
Attachment: Experimental study on the effects of reheat temperatures on the ammonium 01-1676001363-12052.pdf (4.6MB)
This file has been downloaded 59 times
[Edited on 6-10-2025 by pots-o-potash]
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pesco
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Gypsum (plaster of Paris) diaphragm works fine. I have been pretty much soaking rockwool with gypsum with a dash of PVA to make mine. The rockwool for
structural strenghth and PVA as a glue. Not sure if PVA is necessary, but for sure too much PVA hinders permeability. Works OK for MgSO4
electrolysis but tried only briefly, so can't say much about long term durability.
It was just an ad hoc experiment.
Probably won't work for CaSO4 electrolysis.
Obtaining sulphur is worthwhile endeavour on its own. Just in case, at some point, everything but NaCl gets banned 
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